Returning to Missouri today, I was greeted by hot, humid air. Having just endured the same conditions across Kansas, it seemed that no relief was in sight, at least for the next few days.
Then, about 5 PM, I heard rumbles of thunder. Checking the local radar, I observed a band of thunderstorms, moving in from the northeast; since fronts almost always arrive from the southwest, west or northwest, I scoured the National weather maps and regional temperatures. While 90s (F) predominated across most of Missouri (exacerbated by dew points in the 70s), temperatures across the northeastern quadrant of the State were in the mid 80s. A cold front, extending across the Northern Plains and Great Lakes region, had buckled southward, undercutting the hot, humid air and igniting a line of thunderstorms from south-central Iowa to east-central Missouri. Torrential rain fell in Columbia, providing temporary relief but sure to boost the humidity in its wake.
Such transient dips in cold fronts are generally produced by disturbances (centers of low pressure) moving along the front from west to east. Ahead of the trough, storms move in from the northwest (as usual) while, on its "back edge" they arrive from the northeast. When it's this hot, we'll take rain from any direction!